Human locomotion, such as walking and running, is commonly described in terms of gait or a gait cycle. Gait is a cyclical or reoccurring pattern of leg and foot movement, rotations, and torques that creates locomotion. Due to the repetitive nature of gait, gait is typically analyzed in terms of percentages of a gait cycle. A gait cycle is defined for a single leg beginning with the initial contact of the foot with a surface such as the ground. The initial contact of the foot on the ground is referred to as a heel strike. The conclusion of a gait cycle occurs when the same foot makes a second heel strike. A gait cycle can be divided into two phases: stance phase and swing phase. Stance phase describes the part of the gait cycle where the foot is in contact with the ground. Stance phase begins with heel strike and ends when the toe of the same foot leaves the ground. Swing phase describes the part of the gait cycle where the foot is in the air and not in contact with the ground. Swing phase begins when the foot leaves contact with the ground and ends with the heel strike of the same foot. For walking gait speed, stance phase typically describes approximately the first 50%-60% of the gait cycle, while swing phase describes approximately the remaining 40%-50% of the gait cycle.
Individuals have unique gait patterns. Energy or metabolic expenditure during an individual's gait depends on several factors including, body mass, stride length, step rate, and other physical and environmental factors. Individuals have physical and metabolic limits, which determine the speed and distance an individual can travel on foot. Injury or disease may affect a person's range of motion or gait efficiency. Carrying heavy loads may also reduce gait efficiency. Carrying significant loads over long distances and time periods can lead to fatigue and cause musculoskeletal injuries. Military personnel are considered particularly at risk for fatigue and injury from carrying loads. As the quantity and complexity of gear used in military duty has increased, the weight of loads carried by military personnel has also increased. Many soldiers carry a variety of devices, such as night goggles, global positioning systems (GPS), body armor, and other gear. Although maximum loads are recommended, the recommended maximums are typically exceeded. Typical loads carried by soldiers can range between 45 kilograms (kg) to 60 kg or more. Soldiers often carry the loads for long distances while marching on foot.
The relationship between distance traveled and the rate of metabolic energy expended is exponential in nature. The metabolic cost of gait depends on the speed of gait, the physical ability of the individual, and the weight of a load carried by the individual. When carrying a heavier load, the speed of a march is decreased in order to avoid fatigue. Fatigue has been shown to have detrimental effects on individuals who carry the heavy loads. Fatigue is known to increase likelihood of acute injury by raising the potential for trips and falls. Fatigue can also affect mental focus, reduce situational awareness, and negatively impact overall physical and mental performance. Non-combat related injuries caused by carrying significant loads are also a problem. Long term and chronic overuse injuries account for a significant amount of injuries for soldiers.
Individuals who lack able-bodied motion or gait may have a reduced range of motion at one or more joints or may be unable to supply the power to move a limb or a joint in an able-bodied path. Orthotic devices help restore mobility to people who lack able-bodied motion or gait. People who require a lower limb orthosis often expend more metabolic power to walk or move at the same speed as able-bodied individuals. One goal of lower limb orthotic devices is to help the user achieve a normal gait while reducing energy expended by the user.
Various types of structures and exoskeletons have been proposed to support gait. The human hip is a three degree-of-freedom joint that possesses a large range of motion. In addition to the kinematic flexibility of the human hip joint, the waist and hip size among individuals varies. Current exoskeletons are limited in ability to adjust to different size individuals or with different gait patterns. One example of current joint augmentation system for the hip supports only uni-directional motion. Other joint assistance structures interfere with the range of motion of the human joint, resulting in limited usefulness in combat conditions. Weight of wearable exoskeletons and wearable orthotics is also a concern. Joint assistance systems that are too heavy diminish or negate any joint assistance provided by the system.